The disclosure provides methods for identifying a PAR4 agonist responder or a PAR4 antagonist responder comprising contacting a PAR4 agonist with platelets in a sample obtained from a subject in need thereof. The methods also include administering a PAR4 antagonist to the subject identified as a PAR4 agonist responder or a PAR4 antagonist responder. The invention also includes methods of treating the subjects and a pharmaceutical kit comprising a PAR4 agonist.